An optical waveguide device using an electro-optic crystal such as lithium niobate (LiNbO3) or lithium tantalate (LiTaO3) is formed by forming a metal film, such as titanium (Ti), on a part on a crystal substrate, and then thermally diffusing, or proton-exchanging in benzoic acid after patterning, to thereby form an optical waveguide, and then providing an electrode near the optical waveguide. As such an optical waveguide device using an electro-optic crystal, for example, an optical modulator as illustrated in FIG. 14 is known.
In FIG. 14, an optical waveguide formed on a substrate 100 includes; an input waveguide 101, a pair of branched waveguides 102 and 103, and an output waveguide 104. On the pair of branched waveguides 102 and 103, a signal electrode 105 and a ground electrode 106 are provided to form a coplanar electrode. In the case where a Z-cut substrate is used, the signal electrode 105 and the ground electrode 106 are arranged immediately above the optical waveguide, in order to use the refractive index change due to an electric field in the Z direction. More specifically, the signal electrode 105 is patterned on the branched waveguide 102 and the ground electrode 106 is patterned on the branched waveguide 103. Here, in order to prevent light propagating in the branched waveguides 102 and 103 from being absorbed by the signal electrode 105 and the ground electrode 106, a buffer layer (not illustrated in the drawing) is provided between the substrate 100, and the signal electrode 105 and the ground electrode 106. As the buffer layer, silicon oxide (SiO2) or the like having a thickness of from 0.2 to 2 (μm) is used.
In the case where such an optical modulator is driven at high speed, an output terminal 105OUT of the signal electrode 105 is connected to the ground electrode 106 via a resistance (not illustrated in the drawing) to give a traveling wave electrode, and a microwave electric signal is applied from an input terminal 105IN of the signal electrode 105. At this time, the refractive indices of the branched waveguides 102 and 103 respectively change to such as +na and −nb due to an electric field generated between the signal electrode 105 and the ground electrode 106, to thereby change the phase difference of light propagating on the branched waveguides 102 and 103. Therefore, intensity-modulated signal light is output from the output waveguide 104 due to Mach-Zehnder interference.
An effective refractive index of the microwave electric signal can be controlled by changing a cross-sectional shape of the signal electrode 105, and a high-speed optical responsive characteristic can be obtained by matching the speeds of the light and the microwave electric signal.
It has been proposed to use two such optical modulators and input the output beams thereof to a polarization beam combiner (PBC) as illustrated in FIG. 15, so that the planes of polarization of the respective output beams are orthogonal to each other, to thereby constitute a transmitter for polarization multiplexed communication. If this proposal is further developed and the two optical modulators and the PBC are formed on one substrate and made as a chip, the respective optical modulators and the PBC need not be connected to each other by an optical fiber, and the size of the transmitter can be reduced.
However, since the above-described optical modulator normally modulates TM-mode light having excellent modulation efficiency, the plane of polarization of the output beam of one optical modulator needs to be rotated by about 90°, in order to input the output beams of the two optical modulators into the PBC with their planes of polarization orthogonal to each other. Therefore, it can be considered to form a groove for cutting the output waveguide of one optical modulator, and insert a wave plate into the formed groove.
As a technique for forming the groove for cutting the optical waveguide, one using dicing has been heretofore known (refer to Japanese Laid-open Patent Publication Nos. 07-56199 and 2000-121850).
In formation of the groove by the conventional dicing, as illustrated in FIG. 16, a bottom face of the substrate 100 having the optical waveguides (101 to 104) formed on a top face thereof is fixed on a supporting base 4 such as a cutting table, and a dicing blade 5 is advanced from a side face of the substrate 100.
However, in this method there is the following problem. That is to say, in the case where the two optical modulators are made as one chip as described above, the output waveguides of the two optical modulators are adjacent to each other. Therefore, in the formation of the groove by the conventional dicing process, when a groove for cutting only the output waveguide of one optical modulator is to be formed, the output waveguide of the other optical modulator may be erroneously subjected to damage such as cutting. Consequently there remains a problem in productivity of the optical waveguide device in that the yield of the dicing process (groove forming process) may be worsened, high accuracy is required for dicing, and the like.
Such a problem is not limited to the case where the two optical modulators or the like are made as one chip as mentioned above, and can be said to be one that commonly occurs in the case of forming a groove for cutting only one optical waveguide of two optical waveguides formed substantially in parallel on the substrate.